Is Buffer class thread-safe?

[glcrazier]

When I read source code of skywalking agent core, I notice that the Buffer class doesn't have any synchronization on object array read and write operations. Would it be a problem if the buffer is heavily written by different threads? Would it be better to use ArrayBlockingQueue to implement the same functionality without blocking access?

source code link: org/apache/skywalking/apm/commons/datacarrier/buffer/Buffer.java

[wu-sheng]

I don't know what do you mean not thread safe. They are safe.

[glcrazier]

If write thread switches inside save method and read thread clears buffers, it has possibilities that the buffer is empty but save method returns false which cause to discard trace segments. Is it expected?

[wu-sheng]

Why buffer's slot can't be identified correctly? What causes it?

[glcrazier]

Below is save method:

    public boolean save(T data) {
        int i = index.getAndIncrement();
        if (buffer[i] != null) {
// if thread switches here
            switch (strategy) {
                case IF_POSSIBLE:
                    return false;
                default:
            }
        }
        buffer[i] = data;
        return true;
    }

obtain method:

    void obtain(List<T> consumeList, int start, int end) {
        for (int i = start; i < end; i++) {
            if (buffer[i] != null) {
                consumeList.add((T) buffer[i]);
                buffer[i] = null;
            }
        }
    }

If thread switches after identifying a slot is occupied in save method and obtain method resets all slots, then save method continues executing as the data is dropped which in fact the slot is available at this moment. Is my understanding correct?

[wu-sheng]

Yes, and that is expected. The throughput has higher priority, we don't want to lock. It would be a disaster to agent.

[glcrazier]

Thanks for your reply~